# How to tell if the moon is waxing or waning?

Imagine you are placed in an unknown location in the middle of the ocean. You see the moon at its half.

Which are the different "sets of information" which, if provided to you, would allow you to tell correctly if the moon is waxing or waning?

As you might know, there are several different, independent methods to answer the above based on different space-time information. From my rudimentary/amateur knowledge of astronomy I have identified three. I am, however, curious of all the alternatives.

I am particularly interested in two scenarios:

1. You cannot see the landscape of the surface of the moon (say, a thin layer of clouds cover it).
2. You can see the landscape of the surface of the moon (i.e. you can see half of the "moon rabbit")
• You mean besides the obvious bit of what the moon looks like? Kind of seems like homework... Oct 19 '18 at 15:32
• @JonCuster Definitely not homework, trust me. Just curiosity. What do you mean by "what the moon looks like"? That probably refers to point 2. But what about point 1, where you do not see how the moon looks like? Oct 19 '18 at 15:35
• Even with a thin layer of clouds, the moon looks different between waxing and waning. Consider where the moon and the sun are during the two, and what bits of the moon you will see lit. Oct 19 '18 at 15:36
• @JonCuster If it's nighttime, you do not have the sun as a help. Also, if you are in north hemisphere and look south, position is contrary to that if you are in the south hemisphere, where you look north. In principle you don't know where you are. Oct 19 '18 at 15:44
• It rises to the east and sets to the west. You know E/W, so now the only question is right/left. Sadly, most people don’t spend a lot of time outdoors away from light pollution these days. Oct 19 '18 at 15:52